Question

Describe the acidity and pKa of hydrogen atoms that are bonded to carbon atoms adjacent to alkenes, as compared to hydrogen atoms bonded to tertiary alkanes. What happens to leaving groups that are found in the same adjacent position?

Answer 1

Acidity of H bonded to carbon adjacent to alkene.

We know alkene has double bond and sp2 hybridization. When hybridization is sp2 then the carbon of double bond has more S character than p. So the carbon becomes more electronegative.

This carbon pulls electron density through bond from adjacent carbon and former also becomes more electronegative and so to remove H from it becomes easier.

Now lets look for the H bonded to the tertiary carbon.

In this case the electron density of carbon to which H is bonded is increased through inductive effect due to presence of more Carbon groups around it.

This will prohibit to remove H from it.

And we notice the difference in acidity. In previous case the acidity is more than the later.

When acidity is more then its pka is less than and vice versa.

Now we look for leaving group.

When any leaving groups leaves it forms carbocation. And as we know alkene carbon is more electronegative it decrease electron density on carbocation and makes it more reactive.

Where as more substituted carbon makes stable carbocation.